This invention relates generally to removing impurities in liquid metal such as liquid sodium, and more particularly to a method and an apparatus for efficiently removing impurities in liquid sodium coolant used in a nuclear reactor.
Various impurities such as hydrogen, oxygen, nitrogen, carbon, metallic impurities and the like are contained in liquid metal coolant used in a secondary system or an experimental loop for a fast breeder reactor.
In order to remove such impurities contained in the liquid sodium, there has been used hitherto a cold trap in which the liquid metal is cooled below the saturation temperature of the impurities to be removed thereby precipitating the impurities in the form of various sodium compounds from the liquid sodium. The thus produced precipitate is trapped and accumulated in the cold trap by using, for example, wire gauzes of stainless steel. However, it is pointed out by recent reports that, when hydrogen gas is produced due to a hydrogen diffusion resulted from the corrosion of a heat transfer tube wall in a steam generator, a large quantity of precipitate, such as hydrides of sodium and the like which is especially originated from hydrogen impurity, is separated in the cold trap and such precipitate plugs the cold trap within a period of one or two years. Therefore, it is desirable to remove the impurities, especially hydrogen contained in the liquid metal.
With respect to the removal of hydrogen gas from the liquid metal, it is usually known that hydrogen can permeate through a membrane of nickel, iron or stainless steel which has a corrosion resistance against the liquid metal. Therefore, by providing such metal membrane in a stream path of the liquid metal directly branched from the secondary system or the experimental loop of the reactor and by contacting the liquid metal containing hydrogen impurity with one side of the metal membrane and reducing the pressure of the opposite side of the metal membrane, the hydrogen impurity can selectively permeate in gaseous state through the membrane and can be removed from the liquid metal. Such hydrogen removing system using the metal membrane as described above, however, has been used mainly for determining a hydrogen concentration in the liquid sodium thereby detecting the water leak. For this purpose, it is preferable to maintain the hydrogen concentration in the liquid sodium considerably low (for example about 150.degree. C., when indicated by the saturation temperature of hydrogen) in order to increase the detection sensitivity as much as possible. On the contrary, it is not effective for the purpose of the removal of hydrogen from the liquid sodium to carry out the permeation of hydrogen through the metal membrane under such low saturation temperature. In order to remove a considerable amount of hydrogen from the liquid sodium, a vast area of the metal membrane is required.